The present invention relates to a circuit for detecting deterioration of a laser diode, and particularly to a circuit that detects deterioration of the laser diode based on change of differential efficiency thereof.
In devices employing laser diodes as a light source, it is necessary to monitor operational status of the laser diodes and to detect deterioration thereof in order to ensure the operation of the devices. That is, if the laser diode is deteriorated, even if a predetermined current flows in the laser diode, the optical output power of the laser diode is lower than the designed value.
The main reasons why the laser diode is deteriorated are variation of semiconductor junctions with time, electrostatic destruction of the semiconductor structure, and the like.
A relationship of current to optical output of a laser diode is shown in FIG. 4A. In FIG. 4A, a curve S1 represents a characteristic of a normal (i.e., not deteriorated) laser diode, S2 and S3 represent characteristics of deteriorated ones. As shown in FIG. 4A, when the laser diode is deteriorated, the threshold Ith increases, and differential efficiency (i.e., a ratio of current to optical output power when the current is greater than the threshold Ith) decreases. It should be noted that for stimulated emission, the laser diode should be driven with the current greater than the threshold Ith. In FIG. 4A, thresholds for the characteristics S1-S3 are indicated by Ith(S1), Ith(S2) and Ith(S3), respectively.
The deterioration may be detected based on the threshold Ith. However, the threshold Ith varies depending on the temperature around the laser diode as shown in FIG. 4B. Therefore, the deterioration cannot be detected only by the value of the threshold Ith.
Accordingly, there has been suggested an LD (Laser Diode) deterioration detecting circuit based on differential efficiency thereof. As shown in FIG. 4B, although the threshold Ith varies depending on the temperature, the differential efficiency (i.e., xcex94P/xcex94iL) remains substantially the same regardless of the temperature. While, as shown in FIG. 4A, as the laser diode is deteriorated, the differential efficiency becomes lower. Therefore, based on the differential efficiency, the deterioration of the laser diode can be detected regardless of the temperature.
FIG. 5 shows a block diagram of an example of a conventional LD deterioration detection circuit. The circuit shown in FIG. 5 is a redrawing of a circuit disclosed in Japanese Patent Publication HEI8-4163. In this LD deterioration detection circuit, a laser beam emitted by a laser diode 201 is received by a photo diode 202, which generates a photocurrent ID corresponding to the intensity of the received laser beam. An I-V (current-to-voltage) converter 203 converts the photocurrent ID into a voltage VL, which is input to an inverted input terminal of a differential amplifier 204. The differential amplifier 204 outputs an amplified value of a difference between the voltage VL and a reference voltage VO generated by a reference voltage generator 205. The output of the differential amplifier 204 is input to a hold circuit 206, output of which controls a drive current supply circuit 207 to output a driving current IX for driving the laser diode 201.
When a deterioration detection command signal E is OFF (not issued), the circuit performs an APC (Automatic Power Control) process, and the current IX is controlled such that the voltage VL output by the I-V converter 203 is equal to the reference voltage VO.
When the detection command signal E is ON (issued), the circuit performs the detection of deterioration of the laser diode 201. That is, when the command signal E is ON, a switch 208 of the hold circuit 206 is switched such that the output of the differential amplifier 204 is not transmitted to the drive current supply circuit 207. The hold circuit 206 holds the output voltage of the differential amplifier 204 just before the switch 206 is turned OFF with a capacitor 213, thereby the driving current IX is maintained as a fixed value. Further, when the command signal E is ON, a switch 209 is switched so that a predetermined current xcex94I from a biasing circuit 210 is added to the drive current IX and then supplied to the laser diode 201.
A deterioration determining circuit 211, which includes a comparison circuit and an inverting circuit, compares the voltage VL output by the I-V converter 203 with a deterioration detection reference voltage VT for determining whether the laser diode 201 is deteriorated. The comparison result of the deterioration determining circuit 211 and the command signal E are applied to an AND gate 212, which outputs a deterioration detection signal Sout.
In this conventional circuit, in accordance with the detection command E (when the command E is ON), the current xcex94l from the biasing circuit 210 is added to the driving current IX, which flows in the laser diode 201, thereby output power of the laser diode 201 temporarily increases. The output photocurrent ID of the photo diode 202 increases, and accordingly, the output voltage VL of the I-V converter 203 also increases. The deterioration determining circuit 211 compares the output voltage VL with the reference voltage VT. If the voltage VL is greater than the reference voltage VT, it is determined that the differential efficiency of the laser diode 201 is greater than a predetermined value, and thus, the deterioration detection signal Sout will not be output. If the voltage VL is not greater than the deterioration detection voltage VT, it is detected that the differential efficiency has decreased, and therefore, the deterioration detecting signal Sout is output.
In the conventional LD deterioration detecting circuit as described above, the driving current IX is held by the hold circuit 206, and then a current xcex94I from the biasing circuit 210 is added to the driving current IX. Then the sum of the current (IX+xcex94I) is supplied to the laser diode 201. Thus, the voltage VL corresponding to the current (IX+xcex94I) is output by the I-V converter 203, which voltage VL is compared with the reference voltage VT by the deterioration detecting circuit 211. If the optical output power of the laser beam is to be selectively set to one of a plurality of levels (either stepwise or continuously), it should be ensured that the optical output of the laser diode 201 is adjusted to correspond to the reference voltage VO, which is the reference voltage for examining the laser diode 201, before the deterioration detection command E is issued. Therefore, control is complicated.
Further, since the hold circuit 206 is connected in series with the APC circuit, when the deterioration detection is not performed, the sample-hold circuit 206 composed of the switch 208 and the capacitor 213 functions as a low-pass filter, which lowers a frequency characteristic of the APC. Specifically, the APC functions to maintain the optical output of the laser diode 201 to be predetermined quantity when it is driven. At the time when the laser diode 201 is started to be driven, or when the optical output is changed and then returned to the predetermined quantity, the frequency characteristic of the APC affects the response of the APC system. Generally, if the frequency characteristic becomes higher, the response of the APC system is improved.
Furthermore, since the deterioration detection circuit 211 continuously compares the voltage VL with the reference voltage VT, even if the detection command E is OFF, i.e., even when the bias current xcex94I is not added to the drive current IX, if the voltage VL temporarily increases and exceeds the reference voltage VT, the detection signal Sout is issued. In order to avoid such a misdetection, the AND gate 212, which prevents output of the detection signal Sout when the detection command E is OFF, must be provided, and therefore the circuit configuration is complicated.
It is therefore an object of the present invention to provide an improved deterioration detection circuit for a laser diode, which is capable of detecting deterioration regardless of the output power of the laser diode before a detection command is issued, which does not affect the frequency characteristic of the APC circuit, and which has a relatively simple configuration.
For the above object, according to the present invention, there is provided a laser diode deterioration detection circuit for a laser beam emitting device employing a laser diode. The laser beam emitting device includes a drive current supply circuit that supplies a driving current to the laser diode, a laser power detection circuit that detects an optical output power of the laser diode and outputs a laser power detection signal representing the detected optical output power, a first differential amplifier that outputs a first differential signal corresponding to a difference between the laser power detection signal and a reference value, the drive current supply circuit varying the driving current based on the output of the first differential amplifier. The laser diode deterioration detection circuit includes a biasing circuit that modifies the laser power detection signal or the reference value upon issuance of deterioration detection command, a sample-hold circuit that samples and holds a value of the first differential signal output by the first differential amplifier before the deterioration detection command is issued, a second differential amplifier that outputs a second differential signal corresponding to a difference between the first differential signal after the deterioration detection command is issued and the value held by the sample-hold circuit, and a deterioration determining circuit that compares the second differential signal and a deterioration detection reference value, the deterioration determining circuit determining whether the laser diode is deteriorated based on the comparison result.
Optionally, the biasing circuit includes a signal source that generates a predetermined modifying signal to be applied to the laser power detection signal or the reference value, an operating element that add/subtract the predetermined modifying signal to/from the output detection signal or the reference value, and a first switch that connects the signal source when the deterioration detection command is issued.
Further optionally, the laser diode deterioration detection circuit may include a second switch that normally connects the sample-hold circuit with an output terminal of the first differential amplifier, the second switch disconnects the sample-hold circuit from the output terminal of the first differential amplifier when the deterioration detection command is issued.
Still optionally, the deterioration detection reference value is determined based on the modifying signal and a differential efficiency characteristic of the laser diode.
Furthermore, the sample-hold circuit, the second differential amplifier and the deterioration determining circuit can be arranged at least out of an automatic power control (APC) loop for maintaining the output power of the laser diode at a predetermined level, the APC loop including the drive current supply circuit, the laser power detection circuit and the first differential amplifier.
With this configuration, part of the circuit which contributes only to the detection of deterioration of the laser diode does not affect the APC performance.